Highway drivers from Virginia to Rhode Island on Monday will begin to see electronic billboards with larger-than-life police sketches of an unidentified man who has been attacking women for more than a decade - images that detectives hope will spark the tip they need to catch an elusive predator they call the East Coast Rapist.

The billboards are the latest effort by law enforcement to identify the man who has sexually assaulted at least 12 women since the late 1990s. His last known rape was in Prince William County on Halloween in 2009, a brazen attack on three teenage girls headed home from a night of trick-or-treating. DNA evidence links the rapes.

Detectives in four states and FBI agents have been searching for the rapist, but they have not been able to name him. They say he is black and about 6 feet tall, and once had a badly chipped tooth. He often wore a ski mask or hat during attacks. He has used a knife, gun, screwdriver and broken bottle to overpower victims.

During the past year, detectives have closely focused on and ruled out more than two dozen men who fit the rapist's description and who have connections to the locations where incidents have occurred since 1997: Prince George's, Fairfax and Prince William counties, Leesburg, New Haven, Conn., and Cranston, R.I.

They have been narrowing their search by reviewing lists of tens of thousands of potential suspects, revisiting neighborhoods, reinterviewing witnesses and, ultimately, surveilling people and collecting their DNA.

Because the rapist has left his DNA behind, police can quickly rule out suspects and will know for certain when they find the attacker.

The new push for tips comes 16 months after the last confirmed attack - a relatively long span for the East Coast Rapist - and police hope to stir up new information with their public appeal.

"We want to put him on notice that these are still active cases, that police are right on his trail," said Fairfax Detective John Kelly. "Maybe he'll stay out of his next crime."

The billboards direct people to visit a new Web site dedicated to the case, www.eastcoastrapist.com. The site provides detailed descriptions of the attacks, posts three composite sketches and gives users a link to report tips or suspicions. Fairfax County detectives said this is the first time they've used anything like it.

Detectives said they hope people will see the billboards, visit the site and provide the one tip that will lead to the rapist's arrest.

"People can really be the detectives, in a sense," said Lt. Bryan Holland of Fairfax's cold-case squad.

Digital billboards in seven states, up and down Interstate 95, will feature the images, according to FBI spokesman Christopher Allen, who said one also will appear in Times Square. He said arrests in 39 cases - including that of the so-called Granddad Bandit bank robber - are attributed to tips from people who contacted authorities after spotting a billboard.